2 Locations of Data Acquisition

For the purpose of AquaSat, surface reflectance data are acquired at specific, centrally-located points within waterbodies and at locations where there are in situ data. The data acquired at locations where there are in situ data are acquired to create location-specific algorithms using the AquaMatch database, which can then be applied to the data collected over the centrally-located point across all waterbodies with surface reflectance data. AquaSat does not acquire nor summarize data over the entire waterbody’s surface, as it is computationally impractical for most large lakes that cross multiple satellite path-rows or tiles. We are planning to investigate the variability differences in varying buffer distances and whole-lake acquisition to support this decision. At this time, the buffer used is a 120m radius of any given data acquisition location.

2.1 Pole of Inaccessibility

The concept of “pole of inaccessibility” (POI) (Stefansson 1920) is used to define the geographic center of a circle with the largest circumference within any complex polygon. The foundational principle is used widely to describe the arctic pole of inaccessiblity, that is the point in the northern arctic circle that is the furthest from land, but has also been used to describe the geographic center of landmasses (Garcia-Castellanos and Lombardo 2007). For lakeSR, we use POI to determine the point in a lake that is furthest from the shoreline using the polylabelr::poi() function (Larsson 2020), which calculates a point in space and the radius of the circle used to define the POI.

2.2 Lakes included in lakeSR

For lakeSR, we use the NHDPlusHR dataset for lakes within the conterminous US and US Territories using the nhdplusTools::download_nhdplushr() function (Blodgett and Johnson 2023). All HUC4s were downloaded and processed on 2023-10-05 using the most updated version available at the time of download. Because the NHDPlusHR dataset is incomplete for Alaska (see below), we used the NHD Best Resolution File for the state of AK (US Geological Survey, n.d.).

NHDPlusHR dataset availability, courtesy of the USGS.

Figure 2.1: NHDPlusHR dataset availability, courtesy of the USGS.

For every HUC4 and the state of Alaska, all waterbodies are limited to those with FCodes belonging to the following groups: 390 (lake/pond), 436 (reservoir), and 361 (playa) and are at least 0.01 km2 (1 hectare) in area according to the area value provided in the NHD file. This filtering resulted in 1,135,312 lakes included in our dataset, including 415,008 from Alaska.

Figure 2.2: Map of POI’s included in lakeSR. Note that the polygons shown on the map are not the NHD polygons and may reflect different geometries than those used to define the POI.

Citations

Blodgett, David, and Mike Johnson. 2023. nhdplusTools: Tools for Accessing and Working with the NHDPlus. Reston, VA: U.S. Geological Survey. https://doi.org/10.5066/P97AS8JD.
Garcia-Castellanos, Daniel, and Umberto Lombardo. 2007. “Poles of Inaccessibility: A Calculation Algorithm for the Remotest Places on Earth.” Scottish Geographical Journal 123 (3): 227–33. https://doi.org/10.1080/14702540801897809.
Larsson, Johan. 2020. Polylabelr: Find the Pole of Inaccessibility (Visual Center) of a Polygon. https://cran.r-project.org/package=polylabelr.
Stefansson, Vilhjalmur. 1920. “The Region of Maximum Inaccessibility in the Arctic.” Geographical Review 10 (3): 167–72. https://doi.org/10.2307/207749.
US Geological Survey. n.d. “USGS National Hydrography Dataset Best Resolution (NHD) - Alaska (Published 20230506) GeoPackage.”